This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Bacterial DNA repair systems serve as a paradigm for understanding nature?s ability to protect genetic information. In particular, the RecA protein is the premier genomic sentinel of E. coli because it plays a crucial role in recombinational DNA repair. This remarkable protein?s activities have been studied extensively over the last twenty years. However, there remain conspicuous gaps in our knowledge. Namely, what is the active structure of the RecA filament and how exactly does the RecA protein interact with DNA? Crystallographic analysis of the RecA protein will enable a comprehensive understanding of the molecular mechanism of recombinational DNA repair. We have crystallized the E. coli RecA protein both in the absence and presence of the activating Mg2+ cation. In contrast to published studies, the preliminary structure from these crystals reveals the structure of (previously) disordered regions some of which are implicated in DNA binding. Furthermore, we have potential co-crystals of the RecA protein bound to ATP analogues that can activate the RecA filament. Synchrotron radiation is necessary for these fragile, radiation sensitive crystals to determine a high-resolution structure of the active RecA conformation.